Can I use two curtain/inlet motors on one actuator input?

The PROPER way to control this situation is to power the motors separately each having its own pair of relays at the control. You would also have a potentiometer for EACH motor, using separate actuator inputs on the control.

WHY ONE ACTUATOR INPUT FOR TWO MOTORS DOESN'T WORK

The example here is where you have two curtain motors that you want to control exactly the same. They both go up and down at the same temperatures.

If you were to power these two motors in parallel using one pair of relays on the control and use only one feedback potentiometer connected to the actuator input on the control, the curtains would eventually get out of sync. This is because the motors will never run at exactly the same speed, resulting in one curtain being open more than the other.